This invention generally relates to tiedown systems, and is specifically concerned with a tiedown system for securing a cask that carries radioactive materials onto the trailer of a semi-tractor trailer.
Tiedown systems for securing such casks to the trailers of semi-tractor trailers are known in the art. In one type of system, the cask has a generally elongated, cylindrical shape and is secured on its side within a cradle assembly that is in turn bolted onto the deck of a trailer. The cask typically has at each of its ends a pair of opposing trunnions for mounting purposes. The cradle assembly includes front and back cradle members mounted in a rectangular frame for supporting the two end portions of the cask when the cask is laid upon its side. Each of the cradle members includes an arcuate recess which is complementary in shape to the outer walls of the cylindrical cask, as well as a pair of opposing turnbuckle assemblies that are linked to the cradle member, and are securable to the opposing trunnions of one end of the cask. In operation, the cask is laid sideways with its two ends received within the arcuate recesses of the two cradle members, respectively. The two pairs of opposing trunnions are oriented parallel with the horizontal so that the turnbuckle assemblies may be slipped over the tops of the two opposing pairs of trunnions and tightened down to forceably secure the cask to the cradle assembly.
Industry standards require both the cask and cradle assembly to be strongly secured to the deck and frame of the trailer of a semi-tractor trailer in order to minimize the probability of cask rupture under accident conditions. Specifically, these standards require that the components of the tiedown system should be capable of sustaining forces equivalent to 1.5 to 2.0 times the weight of the cask and its contents along the vertical, longitudinal and traverse axes of the trailer. As a fully loaded type B cask can weigh close to 50,000 pounds, it is clear that each of the component parts of the tiedown system must possess considerable tensile and shear strength if the industry standards are to be met. Hence, prior art tiedown systems have typically included very strong, unyielding trunnions projecting out of the cask walls, turnbuckle assemblies capable of sustaining large amounts of tensile forces, and strong cradle assemblies whose frames are tightly bolted or otherwise affixed to the decks and frames of their respective trailers.
While such tiedown systems are generally capable of safely transporting type B radioactive waste casks, applicant has observed that problems may arise as a result of the overall structural rigidity of such systems. Specifically, when the wheels of the trailers carrying such casks pass over rough or uneven roads and momentarily assume different heights relative to each other, the substantial weight of the cask bearing down on the deck can cause the deck to undergo a significant degree of torsional flexing relative t its direction of travel. The substantial weight of the cask in combination with such torsional flexing can create momentary but very large stresses between the cask trunnions and the turnbuckle assemblies as the sides of the cradle assembly attempt to move relative to the circumference of the cask to accommodate the torsional flexing of the deck below it. Such stresses can also occur between the bolts that tightly mount the frame of the cradle assembly to the trailer deck. These stresses can be large enough to deform local areas of the cradle assembly where the turnbuckle assemblies are linked to the individual cradle members, and to split or otherwise break the frame or deck of the trailer. Over time, these stresses can also induce potentially dangerous amounts of metal fatigue in both the turnbuckle assemblies, and the bolts or other components rigidly affixing the cradle assembly frame to the trailer deck and frame, thereby jeopardizing the ability of the system to handle the G forces associated with industry standards.
Clearly there is a need for a tiedown system which is capable of complying with the G forces set forth by the industry standards, but yet avoids the generation of potentially damaging and dangerous localized stresses. Ideally, such a tiedown system should further be capable of accommodating the displacements of the cask trunnions and other portions of the tiedown system that occur when the trailer traverses uneven road surfaces while maintaining a constant and safe amount of tiedown force between the cask and the cradle assembly. Finally, it would be desirable if the tiedown system were extremely reliable in construction, simple in design, and easily adaptable for use with prior art cradle and cask type systems.